V160 Stacking - 2 x x670v-48x

I'm trying to stack two x670v-48x's together using two 40Gb ports on each of the VIM4-40G4X modules.

How do you tell the switches to use those ports? default config says 47, 48 are the stacking ports but I want them to use 49, 53 on each switch.

I've been through the documentation for 15.3 CLI references but I feel like theres a command missing allowing me to change the specified ports?

When on the CLI and type in "conf stacking-support stack-ports 1" and then tab for next commands it states the next commands should be to
"selection Select physical port to be used as the stack port" however it doesnt let me put in a physical port?!?!?

It just allows me to type "selection" and then either "native" or "alternate"